Symmetric procedural material
Mix Color

First step is to mask out the surface's point that be affected by the flipping of the coordinates.

In order to get the reference needed we should start by adding a Texture coordinates node and pick the Object coordinates
   type (if you are interest in the local Z)

Then add a Separate XYZ to get the Z component of the coordinates and filter out the values greater than 0 with the help of a Math node



If the origin of the object belongs to the simmetry plane, all we have to do is to flip the input coordinates (Generate type in the shown example) with a Mapping node (notice the -1 value for the Z scale parameter) and mix them with a Mix node whose inpuct factor is the mask we created before.